Did Cleopatra have to fight for her right to be ruler?

Yes. At first she was co-ruler with her brother, but she was ousted and her brother had her exiled. She was in the process of raising an army when Caesar came to Egypt and tried to mediate the dispute. It didn't work and Cleopatra was involved in the Alexandrian War against her brother. After winning the was and her brother's death Cleopatra became the ruler along with her younger brother.